The present invention relates to an electrostatic capacity type liquid state detecting sensor for detecting a state of liquid contained in a container.
The exhaust gases emitted from a diesel vehicle contain nitride oxides (NOx) other than carbon monoxide (CO) and hydrocarbon (HC). In recent years, it has been practiced to reduce the harmful nitride oxides into non-harmful gas. For example, it has been proposed to dispose at the exhaust muffler of the diesel vehicle an NOx selective reduction (SCR) catalyst, while keeping urea water that serves as reduction agent in a container separately installed on the vehicle, and inject the urea water into the catalyst to thereby reduce NOx into a non-harmful gas such as N2. With this system, if the urea water is used up, it becomes impossible to urge NOx to be reduced so that a large amount of NOx is emitted from the vehicle. To solve this problem, there have been proposed various steps such as a step of disposing a sensor in a container containing urea water for detecting the level of the urea water and giving the alarm when the remaining amount of the urea water becomes lower than a predetermined value.
As an example of such a sensor for detecting a water level, there is known an electrostatic capacity type liquid state detecting sensor. The electrostatic capacity type liquid state detecting sensor detects an electrostatic capacity at an intervening space between an outer tubular electrode made of a conductive material and in the form of a thin, long tube and an inner electrode disposed within the outer tubular electrode so as to extend axially therewithin. In case of the electrostatic type liquid level detecting sensor used for measuring the level of the liquid that is electrically conductive as urea water, the inner electrode is formed with an insulation layer on the outer surface with a view to preventing occurrence of a short between the outer tubular electrode and the inner electrode. The electrostatic capacity type liquid state detecting sensor is set in a container so that the axis of the outer tubular electrode extends vertically, i.e., upward and downward of the water level. In case the liquid is electrically conductive, the electrostatic capacity of a sensor portion not dipped in the liquid is determined depending upon an air layer in an intervening space between the outer tubular electrode and the inner electrode and the thickness of the insulating layer of the inner electrode. On the other hand, the electrostatic capacity of a sensor portion dipped in the liquid is determined depending upon the thickness of the insulating layer, since the electrically conductive liquid becomes equal in potential to the outer tubular electrode, and becomes higher in electrostatic capacity than that of the sensor portion not dipped in the liquid. For this reason, as the sensor portion dipped in the liquid increases, the measured electrostatic capacity increases, thus making it possible to detect a variation of the water level.
In case of measurement of the liquid level by means of such an electrostatic capacity type liquid state detecting sensor, if the intervening space is constant, the liquid level and the electrostatic capacity are proportional to each other. Namely, the more uniform the thickness of the insulation layer formed on the outer surface of the inner electrode becomes, the more accurate the detection of the sensor becomes. In the conventional electrostatic capacity type liquid state detecting sensor, the insulation layer is formed by a resin tube that covers the outer surface of the inner electrode such that the insulation layer is uniform in thickness.